Surface mounted four terminal resistor

ABSTRACT

An electrical resistor has a surface mounted four terminal current sensor of a very low resistance value and capable of handling short pulses of high power. It comprises a flat metal late, 1 to 50 mils thick, of an alloy of high electrical resistivity, to which are welded, on two opposite sides, two flat metal plates of very high electrical conductivity which serve as terminations for electrical interconnection. A slot is cut, from the outside edge toward the center, into each of the two termination plates which divides them into a wide pad for connection of current carrying wires and a narrow one for voltage sensing. The depth of the slots is optimized to get the best stability of resistance readings with changing ambient temperature and under influence of the self-heating effect.

This application is based upon the applicants' provisional applicationSer. No. 60/074,570 filed Feb. 13, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to a surface mounting four terminalcurrent sensing resistor of very low ohmic value and high stability.

Surface mounted current sensing resistors have been available for theelectronic market for many years. Their construction comprises a flatplate made of a resistive alloy like the Cu—Mn—Ni alloy onto which areplated lands of high conductivity metal forming the four terminals. Thevoltage-sensing node is set in the resistive alloy.

When applied to ohmic values in the range of a few millohms or less,this construction introduces additional Joule losses due to theresistance between the point of connection of the current carrying wiresand the a/m nodes. This leads to an additional temperature rise andresults in drifts of the measurements.

The primary object of this invention is to provide an improved very lowvalue surface mounted current sensor characterized by high stabilitywhen subjected to high ambient temperatures and to pulses of high power.

A further object of this invention is the provision of a resistor madeof an alloy of high resistivity in order to increase its thermalcapacity.

A still further object of this invention is the provision of a resistorin which the dimensions of the resistive plate are chosen in a way tominimize the length of the trimming cuts thus avoiding hot spots atpoints where the current makes a turn of 180 degrees.

A still further object of this invention is the provision of a resistorwith terminals made of thick, high thermal conductivity material, whichacts also as a heat sink during a power pulse.

A still further object of this invention is the provision of a resistor,which is constructed in a way to be capable of withstanding pulses ofhigh power by choice of materials withstanding high temperatures and byreducing thermal resistance within the resistor.

A further object of this invention is the provision of a resistor whichcan be mass produced by stamping, laser trimming and coating by methodsdescribed in U.S. Pat. No. 5,604,477 and which can receive a high powerrating when cemented to a metal base for soldering to a heat sink.

A still further object of this invention is the provision of a resistorthat has terminals plated, for interconnection either by soldering or bywelding.

SUMMARY OF THE INVENTION

A surface mounted resistor is formed by welding to each side of aresistive strip of Ni—Cr alloy two strips, one narrow and another wide,of a Ni plated high conductivity copper. The thickness and width of theresistive strip are chosen to form a resistance value below but close tothe requested target, and therefore to minimize the extent of posteriorlaser trimming. This composite strip is punched to form individualresistors in a way described in the U.S. Pat. No. 5,604,477, but with anadditional slot in the terminations in order to divide them intodistinct current and sense pads, the current pad being at least twice aslong as the sense pad. The depth of the slots is optimized to get thebest stability of resistance readings with changing ambient temperatureand under influence of the self-heating effect. The punched resistorsremain attached to the wide copper strip by one current pad. Thisconfiguration permits four terminal (Kelvin) measurements of resistorson a continuous strip during subsequent trimming operation.

Solder coating is applied to the pads in case the application calls forinterconnection by soldering.

When the intended interconnection is by ultrasonic bonding of aluminumwires, the Nickel coating applied before welding the strips serves thispurpose. Next, the resistors are cut off the strip.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the device of this invention;

FIG. 2 is an end elevational view;

FIG. 3 is a top plan view;

FIG. 4 is a top plan view of a punched wide copper strip, and

FIG. 5 is a side elevational view of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The numeral 10 designates the resistor of this invention. It includes aresistor plate 12 with a pair of pads 14 secured thereto. Each pad has acurrent pad portion 16 and a sense pad portion 18. Resistor 10 isadapted for mounting on substrate 20. Specifically, the surface mountedresistor 10 is formed by welding to each side of the resistive strip 12of Ni—Cr alloy two strips 14, one narrow and another wide, of a Niplated high conductivity copper. The thickness and width of theresistive strips 12 are chosen to form a resistance value below butclose to the requested target, and therefore to minimize the extent ofposterior laser trimming. This composite strip is punched on lines 22(FIG. 4) to form individual resistors 10 in a way described in the U.S.Pat. No. 5,604,477 (incorporated herein by reference), but with anadditional slot 24 in the terminations in order to divide them intodistinct current and sense pads, the current pad 16 being at least twiceas long as the sense pad 18. The depth of the slots is optimized to getthe best stability of resistance readings with changing ambienttemperature and under influence of the self-heating effect. One currentpad 16 of the punched resistors remains attached to the wide copperstrip 26. This strip 26 configuration permits four terminal (Kelvin)measurements of resistors on a continuous strip during subsequenttrimming operation.

As previously indicated, solder coating is applied to the pads in casethe application calls for interconnection by soldering.

When the intended interconnection is by ultrasonic bonding of aluminumwires, the Nickel coating applied before welding the strips serves thispurpose. Next, the resistors 10 are cut off of the strip 26 on lines 14.

In case the application calls for mechanical assembly by soldering thedevice to a metal substrate 20, the resistors 10 are bonded withelectrically insulating cement of high thermal conductivity to a metalbase. The bottom of the base may be plated with nickel and gold forbetter solderability to the substrate.

The layers of resistor 10 are secured together with a high thermalconductivity dielectric cement, such as ceramic powder filled hightemperature cements. Use of beryllium oxide in such cements is acomponent that functions well.

1. A surface mounted four terminal connection current sensing resistorfor Kelvin type measurement comprising, ; a flat metal plate made of aresistive alloy having opposite side surface portions, a pair of highconductivity conductive metal terminal plates each secured welded to aseparate side of the resistance plate with a high thermal conductivedielectric cement and supported by the resistive plate, a slot insertedcut transversely in the terminal plates creating four separate padportions, said slot set to a partial depth that determines the beststability of resistance for sensed voltage drop over the resistor, saidpad portions terminal plate being split into a current pad and a sensepad with each pad portion comprising terminal connection areas; saidcurrent pad portion having a length greater in a direction from saidslot than the corresponding length of said sense pad portion, said sensepad portions being enabling a measurement of voltage drop proportionalto the value of current flowing between the said current pads andresistive to drifts in electrical measurements created by temperaturerises that occur due to pluses of high power or high to changing ambienttemperatures.
 2. A surface mount four terminal resistor having astability of resistance, comprising: a flat metal resistive plate madeof a resistive alloy material having opposite first and second sides; afirst plate of conductive metal attached to and supported by the firstside of the flat metal resistive plate; a second plate of conductivemetal attached to the second side of the flat metal resistive plate; afirst slot cut transversely in the first plate; a second slot cuttransversely in the second plate; the stability of measured resistanceat least partially determined by the depth of each slot; the first slotdividing the first plate of conductive metal into a first sense pad anda first current pad; the second slot dividing the second plate ofconductive metal into a second sense pad and a second current pad; eachof the pads providing a terminal connection area; each of the currentpads having a length greater in a direction from said slot than thecorresponding length of the corresponding sense pad.
 3. The surfacemount four terminal resistor of claim 2 wherein the length of each ofthe current pads is at least twice as long as the corresponding lengthof the corresponding sense pad.
 4. The surface mount four terminalresistor of claim 2 wherein the resistor is bonded to a metal substratewith a dielectric cement for better heat dissipation.